[270] By the end of World War II, the
major properties of liquid hydrogen were well established. The
technique for its liquefaction, first developed by James Dewar in the
nineteenth century, was refined but remained basically the same.
Hydrogen liquefaction equipment and capacity remained small because
the only demand was for research investigations in government
laboratories and universities.

Liquid hydrogen was one of the first liquid
rocket fuels proposed, but it was abandoned because of its low
density, low availability, and handling hazards. Gaseous hydrogen
technology, including its use as a fuel, was developed in association
with dirigibles, but those airships were abandoned. Walter Thiel
experimented with liquid hydrogen-liquid oxygen in a rocket engine in
Germany in the late 1930s, but he experienced difficulties with
leakage. In general, the experiences in using either gaseous or
liquid hydrogen in flight applications were not favorable. To
understand why, let us note the desirable characteristics of rocket
fuels and evaluate hydrogen against each criterion.

High exhaust velocity. This is the single most important performance goal
and is related to the heat of combustion of the fuel and the
molecular mass of the combustion products. The importance of having a
high exhaust velocity was first expressed by Tsiolkovskiy in the
early 1900s. Hydrogen surpasses all other chemical fuels in exhaust
velocity, and were this the only consideration, it would have been
chosen and used in preference to other fuels long ago.

High fuel density. The second most important characteristic is a high
fuel density, for this increases the mass ratio of a vehicle and
increases its range or payload capability. Higher density also
reduces drag during flight through the atmosphere, byallowing smaller and
lighter tanks. Unfortunately, hydrogen has the lowest density of all
fuels, a characteristic most responsible for Tsiolkovskiy's -and
others following him-abandoning the consideration of hydrogen as a
flight fuel. Oberth, however, believed correctly that this handicap
could be overcome by very light construction techniques and by using
hydrogen only in the upper stages of a multistage rocket.

Desirable cooling characteristics.
These include a relatively low
combustion temperature to lessen the heatflow into the engine
walls, and fuel characteristics of high thermal stability and
specific heat so that it can be effectively used as a regenerative
coolant. In addition, a low vapor pressure or low critical pressure
keeps the fuel-coolant from boiling or existing as both liquid and
gas in coolant passages-an [271] undesirable situation. Hydrogen scores well on these
characteristics (except low vapor pressure) but there is no evidence
that anyone considered or experimented with liquid hydrogen as a
coolant prior to 1945-probably because nobody got beyond hydrogen's
undesirable characteristics.*

High reaction rate. Rapid reaction of fuel and oxidizer over a wide range
of conditions is advantageous in converting the energy content of a
chemical fuel to heat in a minimum volume. Hydrogen's high flame
speed, low ignition energy, and wide flammability limits-all
advantages-have been known a long time but were not fully appreciated
until after 1945.

Desirable handling and storage
characteristics. An ideal fuel for
handling and storage has a low vapor pressure, low freezing point,
high shock stability, high ignition energy, and is nontoxic and
noncorrosive. Hydrogen scores poorly on these desirable
characteristics with its very low temperature (high vapor pressure),
low ignition energy, and wide flammability-explosion limits.

Available in quantity. Hydrogen scored low on availability before World War
II primarily because the only demand was for small quantities for
scientific research. Gaseous hydrogen and the technology for its
liquefaction were available, however.

From these six general considerations of
fuels, it can be seen that hydrogen's properties represented the
extremes in both desirable and undesirable characteristics and
offered a fitting challenge to those interested in exploring the
potential of new fuels.

* Robert Goddard has been credited with the idea of
regenerative cooling with liquid hydrogen, but the author questions
this based on research for this book.